Current-resonance-type switching power supplies are known as examples of power supply apparatuses that switch voltage resulting from rectification and smoothing of alternating current voltage input from commercial power supplies (such voltage is hereinafter referred to as input AC voltage) with switching elements to output stable direct current (DC) voltage via insulation transformers.
Such a current-resonance-type switching power supply generally includes a circuit that detects overcurrent at a primary side of the transformer. The overcurrent is detected in order to protect elements including a field effect transistor (FET) serving as the switching element, the transformer, and a current resonance capacitor from an overcurrent state. The switching power supply operates so as to keep the output at a secondary side of the transformer at a constant level with lower AC voltage input from the commercial power supply. As a result, the turning-on time of the FET is increased and the current at the primary side of the transformer is increased to cause the primary side to be in the overcurrent state. When the primary side of the transformer is in the overcurrent state, the current exceeding the rating (breakdown voltage) of the elements including the FET at the primary side possibly flows to damage the elements. Accordingly, it is necessary to monitor and detect the overcurrent state to stop the switching element in order to protect the elements at the primary side.
As a method of detecting the overcurrent at the primary side, PTL 1 proposes a method in which a current detection capacitor connected in parallel to a current resonance capacitor is provided and the current flowing through the current detection capacitor is converted into voltage to detect the overcurrent.